The peculiar feature of known warp machines consists in that the fabric that has just been formed is pulled downwards by means of suitable take-down devices, so that because of friction the old loop is not lifted together with the needle, which is going to take the new yarn, but is left in a lowered position so as to be discharged in its turn from the needle and leave space on the stem for the new loop that has just been formed. Moreover, thanks to the take-down action the loop adhering to the needle is able to rotate the needle latch when said needle moves downwards so as to close the jack carrying new yarn, thus enabling it to discharge the old knitting stitch and to form a new loop (in the case of a latch needle).
As is known, take-down devices comprise at least two rollers, which by means of pressure keep and take down the knitted fabric on its whole length, exerting a force opposed to the upward movement of the needle. As a rule, rollers have a smooth surface, preferably made of rubber, so as not to spoil the fabric undergoing take-down and ensure a uniform traction on the whole piece length. However, rollers actually rotate at a constant speed whereas the piece is formed “jerkily”, i.e. when needles get down and discharge stitches. As a consequence, the fabric is taken down even when no new loops are formed. This imbalance of the force of traction is partly counterbalanced by fabric elasticity, however the pressure exerted by rollers cannot be high so as not to result in permanent fabric deformations. Consequently, when needles get up to take new yarn and force the loop wound on its stem to get up, they are prevented from doing so by the take-down action, though part of the fabric wound on the rollers is pulled upwards because of elasticity and sliding between the rollers.
This phenomenon is very dangerous as far as the fabric-building process is concerned and results in poor quality and even needle breaks when needles do not manage to discharge the old stitch and still take new stitches. In known machines this drawback was solved first by strongly limiting manufacturing speed, since the faster the needle is lifted, the more the loop adheres to its stem since it has not time to slide on the latter. Known machines were then equipped with holding-down elements, which fit in between the needles and prevent the stitch to get up along the stem beyond a given extent. This element is commonly known as “stitch-comb” and is applied under different forms to looms equipped both with latch needles and with compound needles. It should be pointed out that in latch needles the latch is integral with the needle by means of a hinge pin enabling rotation on it, whereas in compound needles needle and latch are separate and are to be moved individually. As is known, a warp machine comprises as many holding-down elements as needles. Each element acts upon a needle basically on the knock-over plane of the needlebed, preferably slightly above it, so as to track down loop welts and prevent them from getting up with the needle. Each holding-down element is further shifted forward so as to keep the loop on the needle stem when the needle begins to be lifted, starting from the lower dead center of its stroke, and stays in this position until the needle has achieved its upward movement. Said elements now get backwards so that the needle gets down and forms a new stitch.
Forces acting on each holding-down element when keeping the stitch in a lowered position are small, about few hectograms and mainly depend on yarn type and knitting density.
Generally, holding-down elements are mounted onto bars (one for single-needlebed machines and two for double-needlebed machines), which are movably connected to the machine frame for instance by means of two arms. However, known machines have some drawbacks.
Warp machines are equipped with up to more than three thousand needles per needlebed, which are arranged on a bar of more than three and a half meters of length and, moving simultaneously, discharge the force of friction setting in with the loops just formed onto the bar carrying in its turn more than three thousand holding-down elements. As a consequence, the total force discharged onto the bar of holding-down elements (which is the sum of the force per needle for the number of needles moving simultaneously) reaches very high values, i.e. some hundreds of kilos. Consequently, bars carrying holding-down elements should be very strong in order to bear such an intense stress.
Moreover, so as to prevent the bar of holding-down elements from bending in its effort to prevent loops from getting up, lever arms are arranged about every half meter, thus increasing machine cost and complexity. Thus, for instance, 8 arms are applied on a 3.5 meter bar if the machine is single-needlebed, whereas 16 are present if it is double-needlebed.
Such oversize of the bar carrying holding-down elements requires a machine with strong and heavy structure so as to support and move these elements at a given speed. However, operating speeds are still small because of the forces of inertia due to the high masses involved. Therefore, beyond strong disadvantages as far as manufacturing costs are concerned, there are also strong disadvantages concerning the final operating speed of the machine.
In order not to load the structure of the bar carrying holding-down elements too much, the take-down system is modified by bringing its pressure between rollers and its rotation speed to the limit which the fabric can bear, which results, as is well known, in risks involving breaks or permanent deformations.